摘要:

AbstractThe13C NMR spectra of twoN‐methylimidazole‐substituted sterol esters are discussed. The calculated shifts are compared to those obtained experimentally. For one of the sterol esters the experimental and calculated data show good agreement, and the substituent increments can be evaluated. This does not apply in the case of the second sterol ester, where the steric and/or electronic effects of a further substituent—a hydroperoxy group in close proximity to theN‐methylimidazole group—prohibits the additive calculation of substituent i

ISSN:0030-4921    

DOI:10.1002/mrc.1270150402

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

Abstract13C{1H} single frequency off‐resonance decoupled spectra of secondary carbons with two chemically non‐equivalent protons are analysed theoretically and experimentally. By a careful analysis of the frequencies of all six transitions all spectral parameters, including the geminal proton‐proton coupling constant, can be determined very accur

ISSN:0030-4921    

DOI:10.1002/mrc.1270150403

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractTypically encountered proton‐decoupled spectra of [U‐13C]‐labeled molecules or proton‐coupled spectra of natural abundance or singly labeled molecules contain many nuclei which satisfy the weak coupling approximation. The spectra of such nuclei are frequently highly asymmetric and often appear to exhibit the skewed intensity distribution characteristic of strongly coupled spins. Analysis of typical cases indicates that such effects arise in the presence of at least one moderately strong coupling interaction in the spin system (Jδν ∼ 1/3), and the apparent intensity asymmetry reflects small differences in the spacing of unresolved components of the observed resonance. This effect is analogous to the case of ‘virtual coupling’ in which weakly coupled spins cannot be analyzed as first order spectra; however; the ABX spin system which generally serves as a model for such systems does not predict the existence of spectral asymmetry for the X resonances. Prediction of this asymmetry requires a spin system of at least four spins with at least ABMX complexity, and can be treated generally using the effective Hamiltonian approach of Poep

ISSN:0030-4921    

DOI:10.1002/mrc.1270150404

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe stereochemistry of various pairs of isomeric 2‐cyclohexen‐1‐ylidenecyanoacetates was assigned using1H NMR spectroscopy. The isomers with the γ‐methylene or the γ‐vinyl protonscisto the carbalkoxy group were found to have the signals of these protons at approximately 0.3 ppm and 1 ppm, respectively, downfield relative to their geometrical isomers or the corresponding 2‐cyclohexen‐1‐ylidenemalononitriles. The observation regarding the γ‐vinyl proton proved useful for the assignment of configuration to cyclohex‐enylidenecyanoacetates derived form cholest‐4‐en‐3‐one. The large and constant downfield shift (c. 1 ppm) of the γ‐vinyl proton whencisto the ester group results from the rigid cyclohexenylidenecyanoacetate system, in which the vinylic proton can approach more closely to the magnetically

ISSN:0030-4921    

DOI:10.1002/mrc.1270150405

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractAn indirect method is employed for determining the15N parameters at the natural abundance level in a series of simple acyclic and cyclic amides. The one bond coupling constant,1J(15N1H), and the15N chemical shift are measured as a function of the carbonyl substituent group or the ring size and the nature of the solvent (CCl4or H2O). These15N parameters are related to the amide bond structure, the nitrogen configuration and possible intermolecular hydrogen bonding (amide‐amide or amide‐wat

ISSN:0030-4921    

DOI:10.1002/mrc.1270150406

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractRotation barriers around the metal—pyridine bond of various macrocyclic complexes of cobalt(III), rhodium(III), zinc(II) and magnesium(II) have been studied using the13C longitudinal relaxation times,T1, of the carbon atoms of pyridine and the macrocycle. The data have been rationalized on the basis of steric and electronic factors. The influence of π back‐donation on the rates of rotation around the axial CoN and RhN bonds is discussed. The agreement found between the conclusions drawn from this study and X‐ray data reported for complexes of identical or closely related structure demonstrates the validity of t

ISSN:0030-4921    

DOI:10.1002/mrc.1270150407

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractProton chemical shifts correlate linearly with charges on hydrogen obtained by a method for iterative partial equalization of orbital electronegativity. The compounds studied comprise a wide variety of classes of molecules; all points fall on a single correlation line, showing the general importance of electronic effects in proton chemical shifts and the physical significance of the atomic charges. On the other hand, as there is no general correlation between charges on carbon and on hydrogen atoms, proton chemical shifts cannot be used as probes for charge densities on carbon atoms. Deviations from the correlation line can be attributed to anisotropy effects and provide an estimate for their magnitude.

ISSN:0030-4921    

DOI:10.1002/mrc.1270150408

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe13C NMR signals of the two methyl groups in 4‐hydroxy‐1‐isopropyl‐2‐oxaadamantane are anisochronous in the presence of Yb(fod)3. This is interpreted in terms of the different populations of three rotational c

ISSN:0030-4921    

DOI:10.1002/mrc.1270150409

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractThe1H and13C spectra of bicyclo[3.1.0]hexanes and thujanes have been recorded and assigned. Application of the Karplus equation has yielded dihedral angles, and a computer calculation of the angle of ring buckle as a function of the main dihedral angles has been carried out. The calculated angles of ring buckle agree well with known values in the bicyclo[3.1.0]hexanes, but for 1‐methylbicyclo[3.1.0]hexanes and thujanes the results are not self consistent. It is suggested that the bridgehead substituent causes the boat to twist, although the twist can be reduced by an axial methyl substituent on C

ISSN:0030-4921    

DOI:10.1002/mrc.1270150410

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY

摘要:

AbstractNitrogen‐14 nuclear quadrupole resonance (NQR) spectra of nicotinic acid derivatives, known as antitumor agents, are reported and analyzed in the framework of the Townes and Dailey theory. A strong correlation exists between the charge difference σNC− π at the pyridinic nitrogen and the inhibition of acetylcholinesterase by nicotinic acid derivatives, in which the inhibition potency (pI50) increases as the charge difference σNC− π increases. There are significant differences between14N NQR spectra for the pyridinic nitrogen sites of nicotinic acid and nicotinamide; these can be explained in terms of the differences in hydrogen bon

ISSN:0030-4921    

DOI:10.1002/mrc.1270150411

出版商:John Wiley&Sons Limited    

年代:1981

数据来源: WILEY